2. Put your hand inside of the bag, and fold the corner where the bottom seam and side seam meet into a triangle. Finger press the seams open and fold flat. (Figure 2)

Figure 2

3. Make sure the side seam is lined up with bottom seam (I stick a pin through the middle of the seam in front of me and keep adjusting the fabric behind until the pin comes out right in the middle of the other seam).

5. Stitch all the way across (taking pins out as you stitch), backstitching at edges. (Figure 4)

Figure 3

Figure 4

6. After you’re sure you’re happy with your gusset, trim away the excess (to avoid bulk in the corners). Repeat at the bag’s other corner. If you’re making a lined bag, you need to make the gusset in the lining in the same way.